As for the recording method by means of an inkjet printer which is one of the typical methods among various color recording methods, various methods for discharging ink have been developed, and in any of the methods, ink droplets are generated and adhered onto various record-receiving materials (such as paper, film and cloth) to perform recording. This method has been rapidly prevailing lately and is expected to continue growing remarkably in the future because of features such as quietness without noise generation due to no direct contact of a recording head with a record-receiving material and as easiness in downsizing, speeding up and colorizing. Conventionally, as an ink for fountain pens, felt-tip pens or the like and as an ink for inkjet recording, inks where a water-soluble dye is dissolved in an aqueous medium have been used, and in these water-based inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an inkjet nozzle. These inks are required to provide recorded images with sufficient density, not to clog at a pen tip or a nozzle, to dry quickly on a record-receiving material, to bleed less, to have excellent storage stability, and so on. In addition, recorded images formed are required to have fastnesses such as water fastness, moisture fastness, light fastness and gas fastness.
Clogging at the nozzle of an inkjet is often due that water in an ink evaporates around of the nozzle before the other solvent and additive do, resulting in the compositional condition that water remains less while the other solvent and additive remain more whereby the coloring matter crystallizes and precipitates. Therefore, it is one of the very important performances required that crystals hardly precipitate even when the ink is dried by evaporation. In addition, for this reason, high solubility in solvents and additives is one of the properties required for coloring matters.
Meanwhile, in order that images or character information on a color display of a computer are recorded in color by an ink jet printer, subtractive color mixture of 4 color inks of yellow (Y), magenta (M), cyan (C) and black (K) is generally used, by which recorded images are expressed in color. In order that images by additive color mixture of red (R), green (G) and blue (B) on CRT (cathode ray tube) displays and the like are, as faithfully as possible, reproduced with images by subtractive color mixture, it is desired that coloring matters to be used for inks, particularly of Y, M and C, have respectively a hue close to each standard and also are vivid. In addition, it is required that the inks are stable in storage for a long period of time, and that images printed as the above have a high concentration and also the images are excellent in fastnesses such as water fastness, moisture fastness, light fastness, and gas fastness.
With the recent development of the inkjet techniques, improvement of inkjet printing speed is remarkable and there is a move to use an inkjet printer, as well as a laser printer using an electronic toner, for document printing on plain paper which is a major application in the office environment. The inkjet printer has some such advantages that there is no need to select the recording paper type and inkjet printers are inexpensive, and therefore it is becoming widespread particularly in small to medium scale office environments such as Small Office Home Office (SOHO). When an inkjet printer is thus used for application of printing on plain paper, there is a tendency that hue, coloring density and water fastness are more emphasized among the qualities required for printed matters. In order to satisfy these performances, a method using a pigment ink has been proposed, but using a pigment ink poses problems such as its own instability and clogging at the head nozzle because the pigment ink is an ink whose coloring matter is not dissolved in a water-based ink and which is in a dispersion state. In addition, when a pigment ink is used, a problem often arises in abrasion resistance, too. Although it is said that dye inks relatively hardly pose such problems, in particular the water fastness thereof is extremely inferior compared with pigment inks, whereby improvement of water fastness thereof is strongly desired. Further, unlike pigment ink, dye ink penetrates rapidly in the depth direction after printed on the surface of plain paper, resulting in that a problem of decrease in coloring density is easily posed.
Many proposals have been made to the problem of improvement of water fastness on plain paper through the ages. As a yellow coloring matter for inkjet which is excellent in water fastness and whose hue and light fastness are improved, for example, a dye described in Patent Literature 1 is proposed. In addition, a yellow dye for inkjet which is good in light fastness and ozone fastness is proposed in Patent Literature 2, where there is no detailed description about water fastness on plain paper.    Patent Literature 1: JP H4-233975 A    Patent Literature 2: JP 2006-152264 A